After reading the initial post I'd like to say that:
-in Crossfire, your overclock is limited to the worst of the 2 cards possible overclock.
-the asus DCII 670 are too expensive for what they give and they are very large while they dont perform better (esp since they dump heat in the case) than other aftermarket coolers (msi etc). Choose another option as the cheapest DCii in the UK is £50 more expensive than over 670 with aftermarket coolers. (found MSI 670 power edition for £70 less than cheapest DCii)
-the 7970 are unarguably better and cheaper in the same time but the MSI lighting model is actually not good in value-performance so u might aswell prefer to buy nVidia at that price. (in the UK overclockers.co.uk offers them at £360 (7 more left atm at this price) casual price is over 400).
-if you cant find cheaper MSI lightinings, maybe try sapphire vaporx 3GB models.
Lastly, if I had to choose between nVidia 670 and AMD 7970 overclocked/aftermarket cards at around the same price (as in your case) i'd pick nVidia mostly for lower power consumption and supposedly better drivers (esp in 2-card configs) although I never owned SLI or Xfire to talk with personal experience.